Bioactivation of Capecitabine in Human Liver: Involvement of the Cytosolic Enzyme on 5′-Deoxy-5-Fluorocytidine Formation

Abstract

ABSTRACT:Capecitabine, an anticancer prodrug, is thought to be biotransformed into active 5-fluorouracil (5-FU) by three enzymes. After oral administration, capecitabine is first metabolized to 5-deoxy-5-fluorocytidine (5-DFCR) by carboxylesterase (CES), then 5-DFCR is converted to 5-deoxy-5-fluorouridine (5-DFUR) by cytidine deaminase. 5-DFUR is activated to 5-FU by thymidine phosphorylase. Although high activities of drug metabolizing enzymes are expressed in human liver, the involvement of the liver in capecitabine metabolism is not fully understood. In this study, the metabolism of capecitabine in human liver was investigated in vitro. 5-DFCR, 5-DFUR, and 5-FU formation from capecitabine were investigated in human liver S9, microsomes, and cytosol in the presence of the inhibitor of dihydropyrimidine dehydrogenase, 5-chloro-2,4-dihydroxypyridine. 5-DFCR, 5-DFUR, and 5-FU were formed from capecitabine in cytosol and in the combination of microsomes and cytosol. Only 5-DFCR formation was detected in microsomes. The apparent K m and V max values of 5-FU formation catalyzed by cytosol alone and in combination with microsomes were 8.1 mM and 106.5 pmol/min/mg protein, and 4.0 mM and 64.0 pmol/min/mg protein, respectively. The interindividual variability in 5-DFCR formation in microsomes and cytosol among 14 human liver samples was 8.3-and 12.3-fold, respectively. Capecitabine seems to be metabolized to 5-FU in human liver. 5-DFCR formation was exhibited in cytosol with large interindividual variability, although CES is located in microsomes in human liver. In the present study, it has been clarified that the cytosolic enzyme would be important in 5-DFCR formation, as is CES.

Preparation of a Specific Monoclonal Antibody against Human UDP-Glucuronosyltransferase (UGT) 1A9 and Evaluation of UGT1A9 Protein Levels in Human Tissues

“…Human liver samples from 14 individual donors were supplied by the National Disease Research Interchange (Philadelphia, PA) through the Human and Animal Bridging Research Organization (Chiba, Japan), and those from 6 Japanese donors were obtained from autopsy materials that were discarded after pathological investigation (Izukawa et al, 2009). Microsomes were prepared as described previously (Tabata et al, 2004). The use of the human livers was approved by the ethics committees of Kanazawa University (Kanazawa, Japan) and Iwate Medical University (Morioka, Japan).…”

Progesterone Receptor Membrane Component 1 Modulates Human Cytochrome P450 Activities in an Isoform-Dependent Manner

“…Human liver samples from nine donors were obtained from the Human and Animal Bridging Research Organization (Chiba, Japan), which is in partnership with the National Disease Research Interchange (Philadelphia, PA). Microsomes were prepared according to the method described previously (Tabata et al, 2004).…”

Involvement of Carboxylesterase 1 and 2 in the Hydrolysis of Mycophenolate Mofetil

“…Because CESs were reported to be expressed in the inner lumen of the endoplasmic reticulum (Robbi and Beaufay, 1987;Medda and Proia, 1992), it was thought that only compounds located in the lumen underwent hydrolysis and glucuronidation. However, Tabata et al (2004a) reported that CES1 is present in both microsomes and cytosols and that cytosolic CES plays an important role in the bioactivation of the prodrug, capecitabine. Because there is approximately 5-fold more total cytosolic protein than microsomal protein, when we extrapolated this to the results obtained in the present study, we estimated that cytosolic MMF hydrolysis was equivalent to approximately 37 or 88% of the microsomal MMF hydrolysis.…”